Generally, research into a secondary battery capable of being charged and discharged unlike a primary battery has been actively conducted in accordance with the development of state-of-the-art fields such as a digital camera, a cellular phone, a laptop computer, a hybrid automobile, and the like. An example of the secondary battery includes a nickel-cadmium battery, a nickel-metal hydride battery, a nickel-hydrogen battery, and a lithium secondary battery. Among them, the lithium secondary battery, which has operating voltage of 3.6 V or more, is used as a power supply of a portable electronic device or a plurality of lithium secondary batteries are connected in series with each other to thereby be used for a high output hybrid automobile. Since this lithium secondary battery has operating voltage three times higher than that of the nickel-cadmium battery or the nickel-metal hydride battery and is more excellent in view of energy density characteristics per unit weight than the nickel-cadmium battery or the nickel-metal hydride battery, the use of the lithium secondary battery has rapidly increased.
The lithium secondary battery may be manufactured in various types. As a typical type of the lithium secondary battery, there are a cylindrical type and a prismatic type that are mainly used for a lithium ion battery. A lithium polymer battery that has been recently spotlighted is manufactured in a pouch type having flexibility, such that it has a relatively free shape.
In this pouch type lithium polymer battery (hereinafter, referred to as a “pouch type secondary battery”), a battery assembly including an anode, a separator, and a cathode is inserted into a pouch type case, and an electrolyte is injected into the pouch type case, and an edge of the pouch type case is then sealed.
Generally, the case of the pouch type secondary battery is made of a metal such as aluminum and a material coated in order prevent corrosion of the metal. When this case is subjected to a cutting process, a metal layer of the case is exposed to the outside, such that metal layer may be corroded. In addition, a local defect or a micro crack generated due to the corrosion of the metal layer at a sealing part sealed after the electrolyte is injected into the pouch type case becomes an electrical path to the outside to cause dielectric breakdown of the battery and becomes a moisture penetration path in the battery to significantly deteriorate sealing strength.